From the Book

Coaxial Cable

Coaxial cable consists of a hollow outer cylindrical conductor that
surrounds a single inner wire made of two conducting elements. One of these
elements, located in the center of the cable, is a copper conductor. Surrounding
the copper conductor is a layer of flexible insulation. Over this insulating
material is a woven copper braid or metallic foil that acts both as the second
wire in the circuit and as a shield for the inner conductor. This second layer,
or shield, can help reduce the amount of outside interference. Covering this
shield is the cable jacket. (See Figure
8-4.)

Coaxial cable supports 10 to 100 Mbps and is relatively inexpensive, although
it is more costly than UTP on a per-unit length. However, coaxial cable can be
cheaper for a physical bus topology because less cable will be needed. Coaxial
cable can be cabled over longer distances than twisted-pair cable. For example,
Ethernet can run approximately 100 meters (328 feet) using twisted-pair cabling.
Using coaxial cable increases this distance to 500m (1640.4 feet).

For LANs, coaxial cable offers several advantages. It can be run with fewer
boosts from repeaters for longer distances between network nodes than either STP
or UTP cable. Repeaters regenerate the signals in a network so that they can
cover greater distances. Coaxial cable is less expensive than fiber-optic cable,
and the technology is well known; it has been used for many years for all types
of data communication.

When working with cable, you need to consider its size. As the thickness, or
diameter, of the cable increases, so does the difficulty in working with it.
Many times cable must be pulled through existing conduits and troughs that are
limited in size. Coaxial cable comes in a variety of sizes. The largest diameter
(1 centimeter [cm]) was specified for use as Ethernet backbone cable because
historically it had greater transmission length and noise-rejection
characteristics. This type of coaxial cable is frequently referred to as
Thicknet. As its nickname suggests, Thicknet cable can be too rigid to
install easily in some situations because of its thickness. The general rule is
that the more difficult the network medium is to install, the more expensive it
is to install. Coaxial cable is more expensive to install than twisted-pair
cable. Thicknet cable is almost never used except for special-purpose
installations.

A connection device known as a vampire tap was used to connect network
devices to Thicknet. The vampire tap then was connected to the computers via a
more flexible cable called the attachment unit interface (AUI). Although this
15-pin cable was still thick and tricky to terminate, it was much easier to work
with than Thicknet.

In the past, coaxial cable with an outside diameter of only 0.35 cm
(sometimes referred to as Thinnet) was used in Ethernet networks. Thinnet
was especially useful for cable installations that required the cable to make
many twists and turns. Because it was easier to install, it was also cheaper to
install. Thus, it was sometimes referred to as Cheapernet. However,
because the outer copper or metallic braid in coaxial cable comprises half the
electrical circuit, special care had to be taken to ensure that it was properly
grounded. Grounding was done by ensuring that a solid electrical connection
existed at both ends of the cable. Frequently, however, installers failed to
properly ground the cable. As a result, poor shield connection was one of the
biggest sources of connection problems in the installation of coaxial cable.
Connection problems resulted in electrical noise, which interfered with signal
transmittal on the networking medium. For this reason, despite its small
diameter, Thinnet no longer is commonly used in Ethernet networks.

The most common connectors used with Thinnet are BNC, short for British Naval
Connector or Bayonet Neill Concelman, connectors (see Figure
8-5). The basic BNC connector is a male type mounted at each end of a cable.
This connector has a center pin connected to the center cable conductor and
a metal tube connected to the outer cable shield. A rotating ring outside the
tube locks the cable to any female connector. BNC T-connectors are female devices
for connecting two cables to a network interface card (NIC). A BNC barrel connector
facilitates connecting two cables together.

Plenum cable is the cable that runs in plenum spaces of a building. In
building construction, a plenum (pronounced PLEH-nuhm, from Latin meaning
"full") is a separate space provided for air circulation for heating,
ventilation, and air-conditioning (sometimes referred to as HVAC), typically in
the space between the structural ceiling and a drop-down ceiling. In buildings
with computer installations, the plenum space often is used to house connecting
communication cables. Because ordinary cable introduces a toxic hazard in the
event of fire, special plenum cabling is required in plenum areas.

In the United States, typical plenum cable sizes are AWG sizes 22 and 24.
Plenum cabling often is made of Teflon and is more expensive than ordinary
cabling. Its outer material is more resistant to flames and, when burning,
produces less smoke than ordinary cabling. Both twisted-pair and coaxial cable
are made in plenum cable versions.